'To jump the gun' definitions:

Definition of 'To jump the gun'

From: GCIDE

Jump \Jump\, v. t.
1. To pass over by means of a spring or leap; to overleap; as, to jump a stream. [1913 Webster]
2. To cause to jump; as, he jumped his horse across the ditch. [1913 Webster]
3. To expose to danger; to risk; to hazard. [Obs.] [1913 Webster]
To jump a body with a dangerous physic. --Shak. [1913 Webster]
4. (Smithwork) (a) To join by a butt weld. (b) To thicken or enlarge by endwise blows; to upset. [1913 Webster]
5. (Quarrying) To bore with a jumper. [1913 Webster]
To jump a claim, to enter upon and take possession of land to which another has acquired a claim by prior entry and occupation. [Western U. S. & Australia] See Claim, n., 3.
To jump one's bail, to abscond while at liberty under bail bonds. [Slang, U. S.]
To jump the gun, to begin to run (in a footrace) before the starting gun has fired; hence, (fig.) to begin any activity before the designated starting time. [1913 Webster +PJC]